A power converter is a device, well known in the art, for converting a DC source voltage, typically unregulated, to a regulated DC voltage for powering a load. The power converter typically has a nominal output value, which is the steady state output voltage generated by the power converter when it is not being adjusted by a user or otherwise affected by short term changes in load demand. It is sometimes desirable to allow the user to adjust the voltage level of the regulated output up or down from the nominal value. One of the applications of a different regulated voltage level is in testing for the existence of race conditions in logic circuits. A race condition is a type of fault in a digital circuit wherein some of the states of the digital circuit could have unpredictable values depending on the propagation delay of the circuit elements in the circuit. One of the ways for detecting the existence of a race condition in a logic circuit is by examining the state of the output while varying the output voltage of the power converter which supplies power to the circuit.
It is also desirable to design a system such that a user can adjust the regulated output voltage easily. The prior art adjustable power converters typically have a nonlinear relationship, such as an exponential relationship, between the adjustment signal and the output voltage. In the case of an exponential relationship, a large initial adjustment signal needs to be applied to the power converter in order to obtain a small deviation from the nominal voltage. When the deviation from the nominal value is large, a small amount of additional adjustment signal would lead to a large change in the output voltage. As a result, it would be difficult for a user to apply the right amount of adjustment signal in order to obtain the desirable regulated output voltage. On the other hand, if the power converter has a linear relationship between the adjustment signal and the output voltage, the same increment in adjustment signal would produce the same variation in output voltage regardless of the extent of deviation of the output voltage from the nominal value. Thus, it is easier for a user to obtain a desired output voltage if the relationship between the adjustment signal and the output voltage is a linear relationship.
FIG. 1 is an example of a conventional power converter 10. Power converter 10 generates a regulated output voltage V.sub.out at a pair of output ports 11, 12. Power converter 10 comprises a power stage 20 for converting DC power from an external voltage source V.sub.in, typically unregulated, to an output DC voltage. Power converter 10 further comprises an error amplifier 32, a reference voltage source 30, and two resistors 24 and 26. The combination of power stage 20, error amplifier 32, reference voltage source 30, and resistors 24 and 26, described below, forms a feedback loop such that the output voltage V.sub.out of power converter 10 is regulated.
Power stage 20 includes a control port 18, an input power port 14 coupled to the external DC voltage source V.sub.in, and an output power port 16 for outputting a voltage which is a function of a signal at control port 18. Output power port 16 is coupled to resistors 24, 26 which are connected in series between ports 11 and 12. Resistors 24, 26 form a voltage divider for generating a comparison voltage at a node 28 so that when the output voltage at ports 11 and 12 is at the nominal value, the comparison voltage is the same as the voltage of reference voltage source 30.
Error amplifier 32 has an inverting input terminal 34, a noninverting input terminal 36, and an output terminal 38. Inverting input terminal 34 is coupled to node 28 and noninverting input terminal 36 is coupled to reference voltage source 30. Output terminal 38 is coupled to control port 18 of power stage 20. As explained below, error amplifier 32 and power stage 20 constitute a controller for generating across output ports 11, 12 a regulated output DC voltage from V.sub.in as a function of the difference between the voltages at input terminals 34, 36.
The operation of power converter 10 is well known in the art. When the output voltage at ports 11, 12 is above its nominal value, the comparison voltage at node 28 is above the voltage of voltage source 30. As a result, the voltage at output terminal 38 of error amplifier 32 is lowered. This lower voltage at output terminal 38, when coupled to control port 18 of power stage 20, reduces the voltage at output power port 16 of power stage 20. As a result, the output voltage at ports 11, 12 is reduced. Similarly, when the output result, the output voltage at ports 11, 12 is reduced. Similarly, when the output voltage at ports 11, 12 is below its nominal value, the comparison voltage at node 28 is below the voltage of voltage reference voltage source 30. Consequently, the voltage at output terminal 38 is raised resulting in an increase in the voltage at output power port 16 of power stage 20. As a result, a higher output voltage is produced at ports 11, 12. As a result of these corrective actions, the voltage at ports 11, 12 is maintained in regulation at the nominal value.
There are several methods for adjusting the voltage level of the regulated output so that it is different from the nominal value. One simple method is to replace the reference voltage source 30 by an adjustable voltage source. By changing the adjustable voltage source to a different value, the potential at node 28, and consequently the voltage level of the output at ports 11 and 12, also will change to a different value. As a result, the voltage level at output ports 11, 12 is maintained in regulation at this different voltage level.
The problem with the method described above is that it may not be possible to replace reference voltage source 30 by an adjustable voltage source. In most power converters, semiconductor integrated circuits are used to reduce the cost and size of the power converters. Typically, such integrated circuits contain an internal error amplifier and an internal reference voltage source coupled to one of the input terminals of the error amplifier. The reference voltage source and the input terminal coupled thereto are thus not accessible outside of the integrated circuit. Consequently, it is usually not possible to replace an internal reference voltage source by an external adjustable voltage source.
Another method for varying the voltage level of the regulated output is to replace one of the resistors 24, 26 by a variable resistor. The problem of this method is that the wiper of a variable resistor, being mechanical in nature, has a tendency to fail. If the wiper of the variable resistor fails, the voltage at output ports 11, 12 could rise to a dangerously high value. The consequence of such an event could be disastrous, because all the circuit elements in an electronic system which are connected to the power converter could be damaged or destroyed.
A common alternative is to place a resistor 39 in parallel with resistor 24, as shown in FIG. 1. The voltage at node 28 can be changed by varying the value of resistor 39. The problem with this method is that the output voltage across ports 11, 12 varies in a non-linear manner with the value of resistor 39. Such a non-linear relationship may confuse the user during voltage adjustment. As a result, the likelihood that a user will make a mistake increases.